Cut off by northern river (6)
I believe the answer is:
severn
'river' is the definition.
(River Severn)
'cut off by northern' is the wordplay.
'cut off' becomes 'sever' (to sever is to cut).
'by' says to put letters next to each other.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'sever'+'n'='SEVERN'
(Other definitions for severn that I've seen before include "British banker" , "Shrewsbury river" , "River at Worcester" , "Longest UK river" , "England's longest river" .)